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From: (Don Wilkins)
Newsgroups: sci.chem
Subject: Re: Yellowing of plastic
Date: Fri, 14 Mar 1997 23:02:21 GMT

On 14 Mar 1997 02:13:08 GMT, (Karol Gajewski)

>Dan Davis ( posted this in sci.chem:
>-=> We on the newsgroups are having a problem with the
>-=> plastic parts of our toys turning yellow, and we don't know why.  Could
>-=> someone here help us?  Here is the current situation:
>-=> Thanks in advance for any help you may be able to give.
>	House paints use lead carbonate (PbCO3) which is white (most 
>hardware stores will mix in the pigments to a bucket of white stuff) ...
>and this can be converted to PbS (which is yellow) and was the 
>explanation that I was given for the yellowing on the outsides of homes..

Not true at all. Pb was replaced with TiO2  years ago.

PbS is black not yellow. 

The paints used to have cadmium pigments and it is cadmium sulfide
which is yellow. The fronts of the old horse drawn streetcars always
turned yellow shortly after being in service. The yellowing on the
outsides of homes was due to cadmium pigments AND being too close to
the barnyard or inconsiderate guests. 

Anyone care to guess the source of the gas which provided the sulfide?

>	Since colouring agents like PbCO3 and CdCO3 are used in 
>combination (PbCO3 has been used in miniblinds, and CdCO3 has been used 
>in teething rings, plastic pigments, etc), I suspect that analogous 
>reactions (i.e. formation of PbS and CdS) are to blame.

I would hope that they would have learned long ago not to put either
Pb or Cd salts in teething rings.

The yellowing of the plastic is more likely the result of insufficient
anti-oxidant present in the plastic. If it is important enough then
storage in the dark under an inert gas is the best solution.

Don't as one poster suggested put them out in the sun.

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